Added checks to make sure the specified track exists

Added precondition checks to validate input files and paths to return more informative error messages as per #2

node_modules/har-schema/lib/request.json

@@ -0,0 +1,57 @@
+{
+  "$id": "request.json#",
+  "$schema": "http://json-schema.org/draft-06/schema#",
+  "type": "object",
+  "required": [
+    "method",
+    "url",
+    "httpVersion",
+    "cookies",
+    "headers",
+    "queryString",
+    "headersSize",
+    "bodySize"
+  ],
+  "properties": {
+    "method": {
+      "type": "string"
+    },
+    "url": {
+      "type": "string",
+      "format": "uri"
+    },
+    "httpVersion": {
+      "type": "string"
+    },
+    "cookies": {
+      "type": "array",
+      "items": {
+        "$ref": "cookie.json#"
+      }
+    },
+    "headers": {
+      "type": "array",
+      "items": {
+        "$ref": "header.json#"
+      }
+    },
+    "queryString": {
+      "type": "array",
+      "items": {
+        "$ref": "query.json#"
+      }
+    },
+    "postData": {
+      "$ref": "postData.json#"
+    },
+    "headersSize": {
+      "type": "integer"
+    },
+    "bodySize": {
+      "type": "integer"
+    },
+    "comment": {
+      "type": "string"
+    }
+  }
+}